In recent years it has become evident that ion gradient-driven cotransport systems play an essential role in bacterial transport and in the function of a variety of epithelia. We propose to organize a conference on the Biochemistry, Biophysics and Physiology of cotransport systems driven by ion gradients across the plasma membrane of bacteria and mammalian cells. Particular emphasis will be placed on cotransport systems driven by proton gradients (mainly prokaryotic) and on cotransport systems driven by sodium gradients (mainly eukaryotic). The specific aim of the conference is to compare in detail what is known about the mechanisms operating in these transport systems because evidence is accumulating that indeed the basic mechanism of energy conversion is quite similar in +H cotransport and Na+ contransport system. An understanding of the mechanism of cotransport is important not only from a biophysical point of view but also has broad consequences for processes related to mitochondrial energy production, regulation of cellular pH and Ca, neural transmission, and absorption and secretion in liver, kidney and intestine, to name just a few. Therefore a description of the normal physiological function will lead to a better definition of the sites where transport systems can be regulated and/or how the systems are affected in disease processes.